Heretofore various systems have been studied and developed as communication systems. As one of them there is known the spread spectrum communication system (hereinbelow called simply SS communication system) described e.g. in U.S. Pat. No. 4,866,734, U.S. Pat. No. 4,899,364, JP-A-Hei 2-69033 (corresponds to U.S. Pat. No. 4 993 044), etc.
In this SS communication system, on the transmitter side, a signal such as narrow band data, voice, etc. is spread in a wide band spectrum by means of a pseudo noise code (PN code) to be transmitter and on the receiver side the signal is reproduced by inversely spreading the wide band signal in the original narrow band signal by means of a correlator.
It is known that this SS communication system is resistant to external interference, noise, etc. (these are called mixed waves) and it has an excluding power corresponding to process gain (PG). The process gain used here can be given by a following formula; ##EQU1## where the radio frequency band width described above means the band width of the transmitted SS signal and the information speed is a data speed in the base band channel.
As such an SS communication receiving device (hereinbelow called simply receiver) using the SS communication system resistant to mixed waves there is known a system disclosed in Japanese patent application No. Hei 1 (1989)-29538 published as (JP-A-Hei 2-207630, and corresponding to U.S. Pat. No. 5,048,052).
FIG. 6 shows the construction of a receiver by the system disclosed in the older application described above. In the figure, reference numeral 30 is a receiving antenna; 31, 36, 42, 42' are band pass filters; 32, 43, 43' are amplifiers; 33 is a local oscillator; 34, 40, 40' are mixers; 35 is an AGC amplifier; 37, 37' are PN code generators; 38 is a clock generator; 39 is an oscillator; 41, 41' are correlators (e.g. surface acoustic wave convolvers); 44, 44' are envelope detectors; 45, 45' are comparators; and 47 is a comparing demodulator.
The receiver by the system disclosed in the older application has been proposed in order to improve data demodulation characteristics by means of the correlators 41, 41', in which IF carrier signals BPSK-modulated with PN codes PN.sub.1 and PN.sub.2 are inputted in the correlators 41 and 41', respectively, as reference signals. In this way, since the PN code of the transmitter is outputted from the correlator 41 in the time region of PN.sub.1 and from the correlator 41' in the time region of PN.sub.2 in the form of respective correlation peak trains.
In a signal received by the receiver having the construction described above, if mixed waves are present in the frequency band of the SS signal, since spurious noise is superposed on the correlation spikes in the outputs of the correlators (outputs of the SAW convolvers), variations are produced in the correlation peaks and the spurious noise. Here the correlation peaks represent results of convolution integration of the SS signal in the received signal and the SS signal of the reference signal obtained by inverting it in time and the spurious noise represents results of convolution integration of the mixed waves in the received signal and the SS signal of the reference signal, as indicated in FIG. 7.
In such a state, only by using the comparators 45, 45', which serve only for shaping the waveform as by the system disclosed in the older application described above, since neither the level of the correlation peaks nor the level of the spurious noise can be detected, the level set for the threshold for separating the correlation peaks from the spurious noise, as indicated in FIG. 8, becomes uncertain and erroneous judgment can take place.
For this reason, the power of excluding mixed waves, which can be expressed by the process gain, is worsened in these comparators due to the fact that neither the level of the correlation peaks nor the level of the spurious noise can be detected.